Serotonergic and FMRF-amidergic neurons in the nerve ganglia of Haliotis asinina Linnaeus.

ABSTRACT The neurons containing serotonin and FMRF-amide were immunohistochemically localized in the cerebral, pleuropedal and visceral ganglia of Haliotis asinina. The large (~10 x 20 [micro]m in diameter) and medium size (~7 x 10 [micro]m in diameter) neurons of these three ganglia contained immunoreactivities to both antibodies against serotonin and FMRF-amide, whereas the more numerous small size neurons (<6 x 8 [micro]m in diameter) did not show these immunoreactivities. The large neurons had oval-shaped nuclei containing mostly euchromatin and long cytoplasmic processes, whereas the medium size neurons, which could be neuroseeretory cells, contained round nuclei with patches of heterochromatin and lack processes. The 5-HT immunoreactive cells were concentrated in the upper half of the medial edge of the cerebral ganglion, the edge of ventral and dorsal horns of the pleuropedal ganglion, and the fight latero-ventral edge of the visceral ganglion; whereas the FMRF-amide immunoreactive neurons were concentrated in the dorsal and ventral edges of the cerebral ganglion, the edge of dorsal horn and the dorsal edge of the body of the pleuropedal ganglion, and the left latero-ventral and latero-dorsal edge of the visceral ganglion. The 5-HT cells were most numerous in the cerebral (~70/2,300 cells per section), and the pleuropedal ganglia (~120/2,700 cells per section), whereas they were least numerous in the visceral ganglion (~50/400 cells per section). The FMRF-amide cells were most numerous in the cerebral (~76/2,000 cells per section), and the pleuropedal ganglia (~130/2,500 cells per section), whereas they were least numerous in the visceral ganglion (~60/1,050 cells per section). The present study suggests that serotonin and FMRF-amide are major neurotransmitters in the neural ganglia, with only the large- and medium-sized neurons involved in the production and storage of these two neurotransmitters.

KEY WORDS: Haliotis asinina, serotonin, FMRF-amide, immunohistochemistry, nerve ganglia

INTRODUCTION

Serotonin (5-hydroxytryptamine; 5-HT) and tetrapeptide Phe-Met-Arg-Phe-amide (FMRF-amide) are neurotransmitters that play many important roles in controlling the physiologic and behavioral processes of invertebrates (Kuang et al. 2002, Mercier et al. 2003). In molluscs, 5-HT controls heartbeat (Skelton et al. 1992), locomotion (McClellan et al. 1994, Satterlie & Norekian 1995), feeding (Kupfermann & Weiss 1981), memory and learning (Nelson & Alkon 1997), and reproduction (Ram et al. 1996, Juneja & Koide 1996). FMRF-amide can evoke muscle tension (Schot & Boer 1982, Buckett et al. 1990), modulate cardiac output (Buckett et al. 1990), and reproduction (Lehman & Greenberg 1987). By immunolocalization methods, 5-HT and FMRF-amide have been observed in many ganglia of the central nervous system (CNS) of gastropod molluscs (Audesirk 1985, Elekes 1992, Croll et al. 2001, Fickbohm et al. 2001), especially in the pulmonate snails, Helix pomatia and Helix aspersa (Griffond et al. 1986, Lehman & Price 1987, Hernadi et al. 1989, Elekes & Nassel 1990, Hernadi 1992). Other gastropods that have also been investigated are a sea hare Aplysia kurodai, a land snail Achatina fulica (Fujii & Takeda 1988), an aquatic snail Lymnaea stagnalis (Audesirk 1985), and an opisthobranch Phestilla sibogae (Croll et al. 2001). The 5-HT and FMRF-amide immunoreactive neurons have been identified in the CNS of all these species with specific distribution patterns. For example, in Aplysia kurodai and Achatina fulica, the 5HT-immunoreactive cells and neuropils were observed in all ganglia except the left pleural ganglia. The sizes of these neurons are generally large, and some reach up to 50-70 [micro]m in diameter. FMRF-amide…